Such an optical measuring head, which is known from EP 1 693 665 A1, contains a light source or an optical fiber end piece for sending a light beam through a measuring gas to a photodetector which is arranged in another such optical measuring head. The measuring heads are mounted at diametrically opposed locations to the wall of a gas duct through which the measuring gas flows. Each of the measuring heads has an optical window which separates a main chamber containing the respective active optical component (light source or photodetector) from a prechamber which opens to the gas duct. The main chambers and, if needed, the prechambers are flushed with a purge gas which is provided by a purge gas source. After flushing the main chambers, the purge gas is collected in a compensation cuvette to be irradiated by a compensation light beam. The portion of the purge gas which is fed into the prechambers is discharged into the gas duct. Though the purge gas should be free from the gas component to be measured, atmospheric gas components may leak into the purge gas and interfere with the measurement. Monitoring the collected purge gas in the compensation cuvette allows compensating any offset in the measurement caused by impurities in the purge gas.
From U.S. Pat. No. 6,071,375, a similar measuring head for monitoring optical emissions in a gas phase processing reactor is known where only the prechamber is flushed with a purge gas. The purge gas is provided by a purge gas source and delivered to the prechamber through a gas line. The purge gas may be an inert gas, such as helium, or may be a fraction of the process gas which is passed into the reactor through the prechamber. Preferably, a gas filter is provided in the gas line. The main chamber containing a detector system and a lens system remains unflushed.